compiler: resolve paths in middleware params

[bajtos]

Introduce a convention for specifying relative paths in middleware params: values prefixed with $! and starting with ./ or ../ are resolved relatively to middleware.json.

Example (server/middleware.json)

{
  "files": {
    "loopback#static": {
      "params": "$!../client"
      // resolved to `client`
    },
    "loopback#static": {
      "params": "$!./public"
      // resolved to `server/public`
    }
  }
}

/to @ritch @raymondfeng please review, especially whether the prefix value makes sense to you too. It was inspired by hashbang (#!) used by client-side MVC frameworks.

I have discovered this use case while reworking the template scaffolded by slc loopback as part of strongloop/loopback-workspace#167

[ritch]

Maybe it requires a name eg. $APP_ROOT... Although I don't mind the current proposal. Feel free to move forward with it if you disagree with using a descriptive name.

[bajtos]

Maybe it requires a name eg. $APP_ROOT... Although I don't mind the current proposal. Feel free to move forward with it if you disagree with using a descriptive name.

I expect this feature will be used quite frequently, therefore I prefer to keep the prefix as short as possible.

Also note that the path is relative to the middleware.json file, not to the project root. I.e. in a typical loopback project scaffolded by slc loopback, you have server/middleware.json and paths are relative to server, not to the project root. It seems to me that APP_ROOT can create confusion as many developers may assume that APP_ROOT is the project root.

[raymondfeng]

I'm leaning toward to use typical variable delimiters for JS templates. For example,

<%= __dirname %>
{{ __dirname }}

It will allow us to use existing template engines to parse the templates and inject values.

I'm fine to have a shortcut for this special case with a prefix such as '#!' (why not use #!?).

[bajtos]

I'm fine to have a shortcut for this special case with a prefix such as '#!' (why not use #!?).

• $ denotes a variable/computed value in languages like Bash/Shell, PHP, Perl
• hashbang paths are usually absolute (e.g. #!/home)
• front-end developers may get confused about #! having a different meaning on the server side

[bajtos]

I'm leaning toward to use typical variable delimiters for JS templates.

That may be a good solution for #75 👍

[raymondfeng]

I'm fine to use $! as the shortcut for the __dirname of middleware.json.